1. Field of the Invention
The present invention is related to a method and apparatus for determining the doping profile of a doped semiconductor region, in particular a junction depth thereof using photomodulated optical reflectance measurement techniques.
2. Description of the Related Technology
The ITRS roadmap highlights the precise characterization of ultra-shallow junctions, formed by shallow doping of semiconductor regions, as one of the top challenges for sub-32 nm Si-CMOS technologies. Such a junction is typically characterized by a maximum active doping level N and a junction depth N.
The used physical and electrical analytical techniques for determining the maximum doping level and junction depth, such as secondary ion mass spectrometry (SIMS), spreading resistance profiling (SRP), four-point probe (FPP), or alternative candidates, such as scanning spreading resistance microscopy (SSRM) allow an accurate determination of this junction depth Xj. However these characterization techniques are destructive and quite slow, e.g. as samples have to be prepared, and therefore prevent any in-line measurement.
Photomodulated optical reflectance (PMOR) is a widely used non-destructive and contactless technique to characterize in a qualitative way the doping profile of such a doped semiconductor region. During measurement, a modulated-power pump laser is directed towards the doped semiconductor region to modify the refractive index profile thereof. This refractive index profile can be modified through generation of excess carriers in the sample, also known as the Drude effect, and/or by temperature effects of the sample under study. Simultaneously a probe laser is directed to this doped semiconductor region where it will be reflected depending on the refractive index profile. By coupling the reflected probe laser signal to a lock-in amplifier, only the variations in the reflectivity of the doped semiconductor sample induced by the modulated pump laser are measured.
An example of such PMOR technique is the Therma-Probe® technique (TP) described in “Non-destructive analysis of ultra shallow junctions using thermal wave technology” by Lena Nicolaides et al. in Review of Scientific Instruments, volume 74, number 1, January 2003. The TP technique is a high-modulation-frequency implementation of the PMOR technique. As the phase shift of the modulated reflected probe laser signal with respect to the pump power laser signal proved to be dependent on the semiconductor doping profile, two independent signals can be obtained from the reflected probe laser signal. These independent signals are labeled as I (in phase) and Q (90° phase difference)
In an embodiment of international patent application WO 2006 063809 titled “Method and device for the independent extraction of the carrier concentration level and electrical junction depth in semiconductor substrate” illustrated by FIGS. 14 and 15, the TP method is used to extract the doping level N and the junction depth Xj of such doped semiconductor sample. To this end two sets of correlation curves are established such that by plotting the pair of independent signals (I,Q) obtained on the doped semiconductor region on these correlation curves, first the maximum doping level N and subsequently the junction depth Xj thereof can be determined.
Although the TP technique allows a non-destructive characterization of a semiconductor doping profile, the variations in the Q signal might prove to be close to the noise level of the measurement to provide a sufficiently accurate value of the junction depth.